Chickens molt in the late fall/winter every year excepting first pullet year. Pullets are in constant molt until they are full grown. Hens will slow down or even stop laying for a time and be losing feathers. The full molt cycle will last 3 months. It's the process of growing new feathers each year. Most of their energy and protein from feed is used for this hence less to no egg production.

Each hen is different as to how long the molt will last, if they stop laying all together for a time during molt and if they gradually lose feathers as replacing them or if they have a "hard" molt where they lose a bunch of feathers all at once looking naked before new ones grow in. There is nothing you need to do though some do give extra protein in attempt to speed up the process though there's no real evidence you actually can speed it up, feathers take time to grow.

1) They lose their feathers and regrow nice new feathers once a year after the first year of life. The young chick goes through 3 partial molts in their first year of life.
2) It really depends. I've had a hen molt very slowly, and a hen that lost the majority of her feathers in a week.
3) Again, depends. Usually fall, but I have a hen that started molting 2 weeks ago.
4) Not all at the exact same time, but in the same season it would be more likely to happen.
5) You would switch them from layer feed to grower feed. I never take mine off grower, but it's best to give them lots of protein during the molting process. I would leave them alone. Don't pick them up - this is really uncomfortable for them.

Yes, they do look pretty odd especially in a hard moult. Be careful not to handle them too much during this time b/c it can be painful for them. Poor things, no only do they look like smush they feel pretty lousy as well.